We have discovered a method for plasma etching a carbon-containing silicon oxide film which provides excellent etch profile control, a rapid etch rate of the carbon/containing silicon oxide film, and high selectivity for etching the carbon-containing silicon oxide film preferentially to an overlying photoresist masking material. Then the method of the inention is used, a highter carbon content in the carbon/containing silicon oxide film results in a faster etch rate, at least up to ta carbon content of 20 atomic percent. In particular, the carbon-containgin silicon oxide film results in a faster etch rate, at least up to a carbon content of 20 atomic percent. In particular, the carbon containg silicon oxide film is plama etched using a plama generated from source gas comprising NH3 and CxFy. It is necessary to achieve the proper balance between the relative amounts of NH3 and CxFy in the plasma source gas in order to provide a balance between etch by/product polymer deposition and removal on various surfaces of the substrate being etched. The NH3 gas functions to "clean up" deposited polzmer on the photoresist surface, on the etched surface, and on process chamber surfaces. The atomic ratio of carbon: nitrogen in the plasma source gas typically ranges from about 0.3:1 to about 3:1. We have found that C2F6 and C4F8 provide excellent etch rates during etching of carbon-containing silicon oxide films.